1. Field of the Invention
This invention relates to digital communication systems and more particularly to a means for obtaining bit synchronism for multiple digital time division multiplex signals received at a central switching or collection center from a number of remote subsidiary centers via communication satellite links.
2. Description of the Prior Art
Synchronous communication satellites can be used to link subsidiary centers to central tandem switching centers. For such synchronous communication satellites the perturbations in transmission time delay over the course of a 24 hour period can be of the order of milliseconds. These perturbations are caused by the drift motion of the satellite relative to the ground stations. Such drift motion effects on such perturbations are significant in comparison to a subsidiary center and a central tandem switching center interconnected by terrestial microwave radio links. The perturbations introduced by the transmission media of such terrestial links is in the order of a nanosecond per statute mile. Thus, even such terrestial links which are thousands of miles in length will have symbol duration perturbations in the order of microseconds. These perturbations in the microsecond range can be readily absorbed by elastic buffers of relatively few bits in capacity. For the links using synchronous communication satellites with delay variations in the order of milliseconds, it would be necessary in order to compensate for the delays using conventional techniques, to place elastic buffers having many thousands of bits of capacity at the central tandem switching center. For large tandem centers involving hundreds of incoming lines, elastic buffering can then represent complexity involving significant expense. According to the present invention, means are provided to adjust the bit rates transmitted at the subsidiary centers so that the bit rate received at the central tandem switching center will be held very nearly constant. This is achieved by using relatively smaller bit-capacity elastic buffers at the tandem center as a consequence of the relatively small differences that are manifested from the average bit rate. Thus, synchronization of multiple source TDM (time division multiplex) signals to a common bit clock at the tandem center can be achieved by a system in which sensing for all accesses is placed in the tandem center with relatively simple apparatus in each associated subsidiary center. Accurate synchronization is an essential prerequisite for such functions as digital switching used in time shared signal (multiplexing) processing techniques.
One system for compensating for delay variations due to satellite drift is described in a paper by R. K. Townley and R. C. Davis entitled TDM/TDMA SYNCHRONIZATION which has been published in the IEEE Electronics and Aerospace Systems Convention (EAS-CON) Record 1970, pages 260-265. This system utilizes a buffer located at the earth terminal (ET) to compensate for delay variations. The minimum size of the buffer is indicated to be two bits. The described system, (page 264), however is unduly complex in that the bit clock located at the technical control facility (TCF) is continuously adjusted to maintain the number of bits in the buffer at a nominal value. This continuous adjustment in the control facility (TCF) bit clock requires a high rate of transmission of control signals from the earth terminal (ET) to the control facility (TCF) thereby requiring a large system bandwidth.